An inkjet printing process uses an array of nozzles to deposit droplets of ink at precisely controlled positions on a receiver medium. The process may be used to print text, patterns or images, or a combination of the above. The quality of the print obtained depends in part on the interaction between the ink droplets and the receiver medium and the way in which the latter controls their movement. The inks used are typically an aqueous solution of dyes, with additional components to control evaporation, viscosity and other physical properties. The inks can also be based on pigments rather than dyes and can be carried in an oil-based rather than a water-based vehicle. Typically, the ink colours used are cyan, magenta, yellow and black. For the highest quality printing of images, additional inks are often used. These are typically pale cyan and pale magenta, thus allowing a greater degree of control over the colour while increasing the volume of ink that must be deposited.
There have been two main trends in inkjet technology over the last few years. One has been the capability of providing high quality prints that rival photographs in their appearance. The other is a trend towards faster printing. Both these changes increase the demands on the receiver media. In order to provide a photograph-like appearance, the media must provide excellent control over the absorption of the incoming ink drops, thus retaining the precision with which the drops have been applied. The difficulties are compounded by the trend towards faster printing, as there is less time in which to control the drops, and also less time for the print to stabilise as it comes out of the printer.
It is possible to print onto plain cellulosic paper using inkjet technology. This has the benefit of offering rapid absorption of the ink, so that the prints are dry as soon as they leave the printer. However, because the paper surface is made up of fibres, it is very irregular in nature, and in particular, the fibres combine to form a matrix of capillaries, which rapidly transport the ink in a range of directions, including the plane of the paper itself. This phenomenon is known as “wicking”. There are two consequences of this: the edges of a printed area appear very irregular, as they reflect the irregularities of the paper. In addition, a large proportion of the ink is drawn below the surface of the paper, so that it is obscured by one or more fibres. This effect leads to prints that appear dull and desaturated, as there is a scattering layer of paper fibres obscuring the dyes that make up the image. The image is also easily damaged by exposure to water.
Porous coatings consisting of porous inorganic particles and a polymeric binder can be applied to plain paper in order to smooth out the irregularities of the fibres and provide improved fixing, while still giving rapid absorption of the ink, as exemplified in U.S. Pat. No. 4,877,678. Such papers give greatly improved control of the ink droplets after arrival at the paper surface, because of their uniformity and the ability to prevent the rapid transport of the ink in a few arbitrary directions. They also increase the brightness of the image, because the dye is prevented from penetrating too far below the surface. A bigger increase is available from the use of finer (approximately 1 micrometer) inorganic particles and an organic binder, as in European Patent specification No. 0495430A1. However, the matt finish limits the brightness of the image and is unsuitable for applications such as electronic photography.
In order to make prints on transparent media, e.g. for overhead projection purposes, it is necessary to apply a coating capable of absorbing the necessary volume of liquid onto a transparent support. Such a coating typically consists of a polymer composition that exhibits controlled swelling in water, as exemplified in European Patent specification No. 0009440A1. Because the swelling process is based on diffusion and is therefore slow, it is necessary to print such media slowly in order to allow time for absorption of the ink. If the ink is delivered too quickly, a phenomenon known as “puddling” occurs, in which neighbouring droplets of ink combine on the surface of the print before being absorbed. In this way, the precision of the print is greatly reduced, and the overall appearance suffers. It is quite usual for prints on swelling polymers to be still tacky on exiting the printer, and even for the ink to be capable of transferring onto a piece of plain paper placed in contact. Such coatings can be applied to white substrates (e.g. white polyester film) or to paper that has optionally been made impermeable by prior coating with a polyolefin in order to make glossy prints of much higher brightness than those obtained on plain paper. However, such media must be printed slowly and often require additional drying time after leaving the printer, because they lack the porosity of the absorbent media.
Solutions have been proposed to enable the achievement of a glossy, fast ink absorbing porous structure in two ways, both with inherent processing disadvantages:    (1). It is known in the art to gel an aqueous polymer solution based on a silica dispersion by a gradual chilling process, such that a glossy, porous surface is achieved, as described in European Patent specification No. 0813978A1 and European Patent specification No. 0888904A1. However, this process is slow and requires the use of specialist photographic coating machines. It is believed that in this known process, solidification is driven by cooling, and not by loss of water. Indeed, it is difficult, if not impossible, to achieve the necessary surface flatness that is required for a gloss finish by a normal drying process applied to aqueous polymer solutions based on silica dispersions suitable for forming coatings which rapidly absorb ink.    (2). Coating formulations have also been proposed which are capable of forming dried porous coatings which can be differently processed to give a flat glossy surface, as described in European Patent specification No. 0803375B1. This method involves forming the coating on the substrate, pressing the coated surface against a smooth surface, and finally peeling away the substrate to reveal a flattened glossy surface. Alternatively, the coating may be separately applied to a smooth glossy surface, dried in situ, and then a suitable substrate attached. The coating may be partially dried before application to the flat glossy surface, as described in European Patent specification No. 0685344B1. This assembly may then be peeled away from the smooth casting surface. This latter method disadvantageously involves an additional processing step.
The media produced by both these processes give prints that are dry immediately they leave the printer, are free from puddling and wicking, and have very bright colours. The bright colours appear because the dyes are fixed in the upper layers of the media. These layers are highly transparent and therefore do not obscure the dyes. The glossy surface also allows the full strength of the colours to be appreciated. In the case of (1) above, it is also possible to produce a surface texture.
The media described in (1) and (2) above are particularly suitable for printing of electronic photographs, but suffer from the disadvantage of being difficult to make and therefore expensive. Also, the high gloss of these materials makes them unsuitable for viewing under conditions where reflections from light sources will obscure the image.
Silver halide prints with a surface intermediate between matt and glossy are usually known as “semi-glossy”, “silk” or “satin” finish, and henceforth the finish will be referred to as “satin”. This surface is particularly favoured for viewing under highly reflective conditions. The colours obtained are nearly as bright as those from prints with a glossy finish, and the satin finish is favoured by many photographers on aesthetic grounds. Inkjet receiver media with this type of finish are well known and commercially available. However, these media have a swelling coating of the kind described earlier. They are therefore very slow to dry and are unsuitable for use with fast modern printers.
The media described in (1) may also be provided with a satin finish as described in European Patent specification No. 1044823 A2. This is accomplished by coating onto a substrate that has been sealed with a polyethylene layer that was subsequently embossed or otherwise treated in order to produce the desired profile. Such media show rapid absorption and bright colours, but are expensive to manufacture.
There does not appear to be any standard definition of a satin finish. It is well understood that the gloss measured for a satin finish surface will be intermediate between that of a gloss surface and a matt surface, but a surface with an intermediate gloss would not necessarily be described as satin. Visual inspection of satin surfaces indicates that they have light reflectance properties such that the reflected beam extends over a range of angles, whereas a normal gloss surface concentrates the light in a very narrow range of angles. Conventional glossmeters are designed to accept only a narrow range of angles from the reflected beam, and thus register a low gloss for satin-finish materials.
Inkjet receiver media require not only to meet the appearance criteria discussed above, but also must have appropriate tactile qualities. In particular, the surface should not feel sticky, rough or powdery. The swelling polymer coatings in particular tend to have a sticky feel, unless they are loaded with large filler particles, which then tend to make them feel rough.